No matter how you slice it, break ups are an awful emotional experience to weather. Whether you were dating three weeks or 30 years, there's still an adjustment period where you feel the weighty absence of your former partner at every turn. Even the best, most amicable break ups come with a fair amount of pain, and let's be honest, most break ups don't fall under that umbrella.
Barring violent or physically dangerous situations, I would venture to say one of the most difficult break ups to get through is one you find yourself regretting. It's not uncommon to initiate the break up, rip off the emotional band-aid only to realize down the road what you're missing out on.